Ah yes, i watched that episode too..
Well, the parents must have offended somebody really badly lah, and
they obviously know who's responsible. Also, i dun think its a
hired killer cos such a brutal act and mutilation has to be done by
someone who's in a mad rage, most likely by the person himself
rather than by an assassin.
My guess is that the parents must have been threatened or have had
some dark, dirty secret that they fear would be revealed should
they finger the killer, that's why they kept quiet.
What i found strange was the part about the guy in the yellow
bloodstained shirt holding the knife. 2 witnesses saw him yet the
police felt that he was not involved. Very strange.. 
